The invention relates generally to internal combustion engine controls and more specifically to a control for adjusting the air-fuel ratio of the mixture supplied to the engine by adjusting the pressure of gaseous fuel delivered to the engine carburetor.
The use of the zirconium oxide sensor to sense a quantity of oxygen in the exhaust gases of an internal combustion engine is well known. Since the quantity of oxygen in the exhaust is related to the ratio of air to fuel in the combustion mixture, such a sensor can be utilized to provide a signal representative of this ratio.
In view of the rather recent and increasing emphasis on reducing pollutants from internal combustion engines, especially those in motor vehicles, design of air-fuel ratio controllers employing such sensors is an active area of technology. U.S. Pat. Nos. 2,389,797, 3,738,341 and 3,815,561 describe several such control systems.
Frequently such systems utilize what may be characterized as an analog or proportional control scheme whereby the exhaust gas oxygen level is determined and a servocontrol mechanism adjusts either the air supply, the fuel supply or both supplies to the engine in order to re-establish operation at or near the control set point. Those familiar with automatic control theory will appreciate that operation of such a system exactly at set point, i.e., with no error signal, is, in fact, not the result of the control system at all but is rather the result of external forces at work which balance the system separate and apart from the servo loop mechanism. Approached from another viewpoint, existence of a finite error signal in a proportional control system resulting from operation away from the set point is necessary in order to commence operation of the mechanism which will attempt to rebalance the system.
The problem has not gone unobserved and numerous control systems employing time based corrective features are known. Such thinking has been applied to the air-fuel ratio controller disclosed in U.S. Pat. No. 4,019,474 and other patents. While these devices exhibit apparently improved control characteristics, the increased complexity results in higher cost and less reliable operation. These deficiencies suggest and encourage the investigation and application of alternate control theories.
One alternate control theory comprehends the use of two position control. As those familiar with zirconium oxide oxygen sensors will appreciate, such sensors exhibit a marked change in output as the constitutents of an air-fuel mixture deviate from stoichiometric. This significant, almost discontinuous, change in the output renders control at the stoichiometric ratio straightforward since small deviations from stoichiometric result in substantial output changes. Aside from the disadvantage of equipment cycling, this arrangement effects accurate operation at the stoichiometric mixture inasmuch as considerations of error signal and operation offset from the set point are negligible. If, however, it is desirable to provide air-fuel mixtures and operate an engine under conditions which vary significantly from stoichiometric, difficulties arise since the change in output voltage of the sensor per unit change of the air-fuel ratio, i.e., the slope of the voltage/air-fuel ratio line decreases as the air-fuel ratio diverges from stoichiometric. Operation of a two position control at air-fuel ratios differing greatly from stoichiometric mixtures may therefore prove difficult due to the small signal variation and hysteresis of the overall system.